The present invention broadly relates to a process and aqueous composition for the treatment of metal surfaces, and more particularly, to a process and aqueous acidic treating solution for the treatment of zinc or zinc-aluminum alloys to form a passivate coating thereon thereby enhancing the corrosion resistance thereof.
It is known in the art to treat zinc surfaces such as galvanized hot dipped and electrolytic zinc-coated ferrous metals such as steel with aqueous acidic passivate solutions containing chromium for depositing a chromium-containing passivate coating thereon thereby substantially improving the resistance of the galvanized material to corrosion, discoloration and white rust formation. The term "zinc surfaces" is intended to encompass relatively pure zinc as well as zinc containing minor quantities of alloying elements. An aqueous treating solution, which is known and commercially employed for this purpose, employs chromate ions, boric acid and hydrofluoric acid as the principal ingredients. While such prior art treating solutions have been found satisfactory for treating substantially pure zinc metal surfaces, such treating solutions have now been found as unsatisfactory for the treatment and passivation of zinc-aluminum alloys which are in widespread commercial use as a substitute for conventional zinc-galvanized steel sheet. One such zinc-aluminum alloy is marketed under the brand name Galvalume and nominally contains about 55% by weight aluminum, 43.5% by weight zinc and about 1.5% by weight silicon. It has been observed that conventional passivation treatments of the types heretofore employed on substantially pure zinc-galvanized stock do not effectively prevent rapid darkening and/or discoloration of such zinc-aluminum alloy surfaces in an unpainted condition when exposed to normal atmospheric conditions. Such discoloration and/or darkening of the surface is objectionable.
The present invention provides for a novel composition and process by which such zinc or zinc-aluminum alloy surfaces can be effectively passivated substantially improving their corrosion resistance and inhibiting the tendency of the passivated surfaces to darken and/or discolor during exposure to ambient conditions.